Resolve "Warn tag can be promoted to the feature level"

Closes #106 (closed)

• PrecedenceBasedOptionsBuilder - add hading for the config
• add rule to src/UnderTest.FeatureLint.Common.Rules
• mark the folder as not a Namespace provider
• add config class to same location
• add a property for the new rule to RuleSettings
• add the rule to RuleType
• unit test your rule
• if it is unique in approach, add an acceptance test
• add the rule to the README.md under Rules
• bump nuget version
• add to CHANGELOG